目录不同时，go程序包路径如何工作？

Question

I am doing some light hacking on the rclone project, and I'm somewhat new to go. I have cloned the repository in my home directory, /home/poundifdef/rclone.

When I look at the file, rclone.go, in the top-level directory, I see the following import statement:

package main

import (
  _ "github.com/ncw/rclone/backend/all"
    "github.com/ncw/rclone/cmd"
  _ "github.com/ncw/rclone/cmd/all"
)

My question is this: nowhere in this code, as far as I can tell, do we specify that the github.com/ncs/rclone/* packages are supposed to refer to the local versions of these directories. Nowhere in my filesystem have I used a directory called github.com/ncw. And yet, running go run rclone.go is able to execute this code.

Where is the path github.com/ncw/rclone being mapped to my local directory, given these packages, as named, do not exist?

Answer 1

github.com/ncw/rclone is a module, because it has a file named go.mod at the repository root. Since you have $GOPATH unset, the experimental module feature is enabled.

Where modules are placed in your filesystem is irrelevant. when constructing import paths, all directory names in the module are appended to the module name. The module name is defined in the first line of go.mod, here "github.com/ncw/rclone". So the package in ./cmd has the import path github.com/ncw/rclone/cmd.

Commands such as go build, go install, go test, etc. download all other required packages automatically to $HOME/go/pkg/mod, unless the -mod=vendor flag is specified, in which case the sources in vendor are used.